OBJECTIVES: 1. To break the transmission chain of toxoplasmosis. 2. To mitigate the disease consequences of Toxoplasma infection by vaccination. 3. To develop genetically modified Toxoplasma and study infection and immunity produced by them in cats and non-feline modes. 4. To develop vaccines suitable for cats and non-feline mammals. 5. To study intestinal immunity in cats. 6. To further characterize the oocyst-induced cycle in cats. 7. To further characterize cycles, infection and immunity of other heteroxenous occidia forming tissue cysts. The nature of immunity in cats will be studied after oral and parenteral infection and challenge by mouth. Some of the strains collected will be used as well as mutants of several types. Other mutants are to be developed, particularly those forming no oocysts or defective oocysts. For non-felines, mutants will be selected that show little central nervous tissue affinity.